Initial phase of Omnivore development achieves
10% improvement in fuel consumption compared to
stratified direct injection engines, also with
ultra low emissions. The research signals a
potential paradigm shift with engine ‘upsizing’
for increased fuel economy.

The first testing phase of Lotus Engineering’s
Omnivore variable compression ratio, flex-fuel
direct injection two-stroke engine has been
successfully completed on gasoline. In addition
to exceptional fuel consumption results, the
engine has successfully demonstrated homogenous
charge compression ignition (HCCI) - where the
engine operates without the need for the spark
plug to ignite the fuel and air mixture in the
cylinder - down to extremely light loads.
Traditionally, this has been challenging but
this combustion process results in ultra low
emissions and has been achieved over a wide
range of engine operating conditions, even from
cold start.

The detailed research has so far focused on
lower speed and load conditions that represent a
major proportion of an engine’s operation in a
real world environment. At 2000rpm and up to
approximately 2.7 bar IMEP (Indicated Mean
Effective Pressure), the ISFC (Indicated
Specific Fuel Consumption) achieved is
approximately 10% better than current
spray-guided direct injection, spark ignition
engines. Emissions results are an impressive 20
ppm NOx at less than 2.3 bar load and has
four-stroke-equivalent hydrocarbons and carbon
monoxide emissions.

Simon Wood, Technical Director of Lotus
Engineering said: “These impressive results
represent an important step-forward in Lotus
Engineering’s strategy of developing an array of
more efficient multi-fuel combustion systems.
Omnivore lays the foundations for a novel and
pragmatic vision of a variable compression ratio
engine concept suitable for production. A
multi-cylinder version is practical for a wide
variety of vehicles and offers greatest benefit
to C and D class passenger cars which can take
advantage of the low cost architecture and
significantly improved fuel economy and
emissions. We are continuing our discussions
with other manufacturers and eagerly anticipate
the development of multi-cylinder demonstrations
of this revolutionary engine configuration.”

The Omnivore engine concept achieves wide-range
HCCI combustion and low CO2 emissions through
the application of a simple wide-range variable
compression ratio mechanism, itself facilitated
by the adoption of the two-stroke operating
cycle. Technologies combined in this package are
all synergistic and provide a route to the
efficient use of alternative fuels, accelerating
the displacement of fossil fuels.

Jamie Turner, Chief Engineer of Powertrain
Research at Lotus Engineering said: “The
automotive industry, including Lotus
Engineering, has quite rightly advocated engine
downsizing for four-stroke engines. This is as a
result of the dominance of the four-stroke cycle
in the automotive world and its generation of
throttling losses at part-load, where vehicles
run most of the time. The two-stroke cycle,
conversely, does not suffer from significant
throttling losses and in many ways is a more
natural fit for automotive use. With the
thermodynamic disadvantages of throttling losses
removed, the two-stroke engine is free to be
sized according to its improved part-load fuel
consumption. Downsizing therefore isn’t vital
and, due to the improved light-load efficiency
and emissions performance we see with Omnivore,
this technology approach and ‘upsizing’ could
permit a more efficient engine.”

The initial Omnivore programme has been in
collaboration with Queen’s University Belfast
and Orbital Corporation Limited Australia, with
sponsorship from DEFRA/DECC and DOE NI through
the Renewables Materials LINK programme. Future
work by Lotus Engineering will concentrate on
further investigating the operation on gasoline
and alternative renewable fuels such as ethanol
and methanol, with more in-depth analysis of
specific test points.

Technical Detail

Omnivore Summary

The Omnivore engine concept features an
innovative variable compression ratio system and
uses a two-stroke operating cycle with direct
fuel injection. It is ideally suited to
flex-fuel operation with a higher degree of
optimisation than is possible with existing
four-stroke engines.

The engine concept features a monoblock
construction that blends the cylinder head and
block together eliminating the need for a
cylinder head gasket, improving durability and
reducing weight. In this case, the application
of a monoblock is facilitated by the absence of
the requirement for poppet valves. A novel
charge trapping valve in the exhaust port allows
asymmetric timing of exhaust flow and continuous
variation of the exhaust opening timing.

The variable compression ratio is achieved by
the use of a puck at the top of the combustion
chamber. This simple, yet effective system moves
up and down effecting the change in geometric
compression depending on the load demands on the
engine.

Engine Concept Features

Monoblock

The monoblock incorporates the cylinder head,
the cylinder barrel and the inlet ports,
together with mounts for the variable
compression ratio system and the charge trapping
valve housing. It also contains the non-moving
location of one of the two possible injector
mounting positions provided for research
purposes. The other injector position is in the
variable compression ratio puck. The monoblock
is mounted on the upper crankcase, which is a
common component with all of Lotus’
single-cylinder research engines. The engine
carries a full primary and secondary balancer
system. The monoblock is water-cooled by an
electric water pump.

Computational fluid dynamics is used extensively
to ensure effective cooling of the monoblock, a
feature assisted by the removal of the cylinder
head gasket, inherent in such architecture. The
chief advantage of a monoblock construction in
any engine, aside from the bill of materials and
assembly benefits, is the reduction of bore
distortion afforded by the removal of cylinder
head bolts. This is especially important in
piston-ported 2-stroke engines.

Variable Compression Ratio Mechanism

The primary component of the variable
compression ratio mechanism is what is termed
the ‘puck’, or a moveable junk piston in the
cylinder head. In the case of the research
engine, this puck is driven in and out by a
double-eccentric mechanism itself comprising
proprietary parts. The puck itself does not move
at engine speed. In addition to the spark plug,
the puck carries one of two possible injector
positions. It is water-cooled and carries simple
piston (or ‘junk’) rings for primary sealing,
and an ‘O’-ring towards the top for final
sealing.

The variable compression ratio system is
controlled by an electric motor and worm drive
arrangement at the front of the engine. Because
there are no poppet valves in the engine, it is
clear that the puck could be of a large diameter
and since there is no need for valve cut-outs in
the piston crown, the minimum volume of the
combustion chamber can be much smaller than has
been the case in variable compression ratio
engines shown to date. When the puck is in its
innermost position, its surface is essentially
coincident with that of the combustion chamber
squish band and this yields the highest
compression ratio of 40:1.

The combustion chamber geometry necessarily
alters as the puck is moved to vary the
compression ratio. The chamber geometry in
Omnivore was therefore chosen on the basis of
2-stroke experience in spark ignition operation.
Consequently, the puck is positioned in the
cylinder head in such a way that the non-moving
squish band directs cooling flow towards the
spark plug. The puck is water-cooled from the
main engine cooling circuit.

Charge Trapping Valve

The charge trapping valve is caused to oscillate
by a short articulated connecting link from an
engine-speed eccentric shaft itself rotated by a
belt drive from the crankshaft. A simple charge
trapping valve mechanism provides for asymmetric
exhaust timing and hence a modification of the
original piston-ported two-stroke operating
cycle. Fitting an articulated link between the
eccentric shaft and the trapping valve actuating
arm affords the opportunity independently to
vary the opening and/or closing point. In this
‘variable’ form, at light load, the charge
trapping valve can be made to control exhaust
port opening, to maximize expansion in the
cylinder, and the blowdown period can be
optimised. The position of the control arm is
controlled by the engine management system. All
charge trapping valve components and their
configuration have been analysed kinematically,
and since they operate with modified simple
harmonic motion, they do not suffer from jerk
stresses.

Other Components

The cranktrain of the engine comprises an 86 mm
stroke crankshaft, a trunk piston of 86 mm bore
and a connecting rod with 195.5 mm between
centres. The piston carries four piston rings:
two pegged half-keystone compression rings which
traverse the ports in the upper section, and a
Napier scraper ring and U-Flex oil control ring
in separate grooves in the lower portion. These
are not pegged since they do not have to
traverse the ports. In this manner, the working
chamber is completely sealed from the crankcase
and hence wet-sump lubrication can be employed.

Since this is a research engine, it is cooled by
an electric water pump with a separate
electrically-driven oil pump used for
lubrication. Scavenge air is provided externally.
For convenience, air for the Orbital air-assist
DI system is provided from the factory air
supply regulated to 6.5 bar maximum air delivery
pressure. Note that in any multi-cylinder
application it is envisaged that all these
subsystems would be incorporated into the engine
in the normal manner.